The invention relates generally to rankine cycle systems, and more specifically to a dual reheat rankine cycle system and method thereof.
Many power requirements could benefit from power generation systems that provide low cost energy with minimum environmental impact and that may be readily integrated into existing power grids or rapidly sited as stand-alone units. Combustion engines such as micro-turbines or reciprocating engines generate electricity at lower costs using commonly available fuels such as gasoline, natural gas, and diesel fuel. However, atmospheric emissions such as nitrogen oxides (NOx) and particulates are generated.
One method to generate electricity from the waste heat of a combustion engine without increasing the consumption of fuel or the output of emissions is to apply a bottoming cycle. Bottoming cycles use waste heat from a heat source, such as an engine, and convert that thermal energy into electricity. Rankine cycles are often applied as the bottoming cycle for the heat source. Rankine cycles are also used to generate power from geothermal or industrial waste heat sources. A fundamental organic Rankine cycle includes a turbogenerator, a preheater/boiler, a condenser, and a liquid pump.
Such a cycle may accept waste heat at higher temperatures (e.g. above the boiling point of a working fluid circulated within the cycle) and typically rejects heat at reduced temperature to the ambient air or water. The choice of working fluid determines the temperature range and thermal efficiency characteristics of the cycle.
In one conventional rankine cycle system for higher-temperature and larger-size installations, steam is used as a working fluid. Steam can be heated to higher temperatures, capturing more of the exhaust energy, without breaking down chemically. Conversely, steam poses immense difficulties because of the tendency of steam to corrode cycle components and the requirement that steam be expanded to a near-vacuum condition to optimally deliver embodied energy. The substantially low condenser pressure necessitates not only elaborate means of removing non-condensable gases that leak into the system, but also large, expensive and slow-starting, expander stages and condenser units.
In another conventional rankine cycle system, carbon dioxide is used as a working fluid. Carbon dioxide may be heated super critically to higher temperatures without risk of chemical decomposition. Conversely, carbon dioxide has relatively low critical temperature. The temperature of a heat sink must be somewhat lower than the condensation temperature of carbon dioxide in order for carbon dioxide to be condensed into a liquid phase for pumping. It may not be possible to condense carbon dioxide in many geographical locations if ambient air is employed as a cooling medium for the condenser, since ambient temperatures in such geographical locations routinely exceed critical temperature of carbon dioxide.
It is desirable to have a more effective rankine cycle system and method thereof.